1. Field of the Invention
The present invention generally relates to the connection of electrical terminals to plastic panels having electrically conductive grids thereon. More specifically, the present invention relates to the attachment of electrical terminals to an electrical circuit applied to a plastic substrate in a plastic window system in order to provide such things as defrosting and defogging capabilities to the window system.
2. Description of the Related Technology
Electrically heatable grids have long been used for the defrosting and defogging of windows, particularly backlights of automobiles. Various types of electrically heated windows have been devised and typically include an electrically conductive heating grid located toward either the interior or exterior side of the window. The heating grid typically includes a pair of opposed busbars, between which a series of grid lines extend. During the passing of electric current through the heating grid, the resistance of the grid lines results in the generation of heat. This heat dissipates across the window, subsequently defrosting or defogging of the window. In order to provide electricity to the heating grid, the heating grid is coupled to the electrical system of the automotive vehicle.
To achieve this coupling of the automotive vehicle's electrical system to the heating grid, the busbars of the heating grid have been provided with foil tabs that extend beyond the edges of the window. Terminals of a wire harness terminal, from the vehicle's electrical system, engage the tabs. The terminals are of a variety of constructions, but often include a spring metal contact, encased within a housing. When the housing is attached to a tab, the contact is biased against and into contact with the busbar.
In an alternate construction, bonding pads are integrally formed with the busbars and the terminals from the vehicle's electrical system are soldered directly to the bonding pads.
Each of the above constructions has its known problems and limitations. Illustrative of the limitations of the spring contacts, over the life of the vehicle, the spring contacts may become loose due to fatigue and/or vibration, resulting in a non-working or a poorly working heating grid. With regard to the limitations of a pad bonding construction, the application of too much or too little solder weakens the joint between the terminals and the bonding pad, which may result in the terminal being easily dislodged from the bonding pad itself. Due to the low glass transition temperature of plastics, traditional high temperature solder cannot be used to make robust connections to the busbars 18, 19. The soldering temperatures of such solders are too high and result in damage to the plastic of the panel 14, the coatings, or inks thereon. Unfortunately, the commercially available low temperature solders, and even, electrically conductive adhesives, have unacceptable bonding strengths and or reliability. Connecting with such materials results in the terminals being bonded to the busbar and requiring minimal force, i.e. only 5 or 6 pounds of force (push/pull), applied parallel to the surface of the panel, to remove the terminal. Typically, original equipment manufacturers (OEM) require the connection to withstand forces of significantly greater forces before removal, such as around 30 pounds of force.
In view of the above, it is apparent that improved connection constructions for attaching terminals to the busbars of heating grids, or other electrofunctional materials on plastic window systems are required.